Deriving fault slip histories from cosmogenic exposure ages along bedrock fault scarps.
Cosmogenic surface exposure dating is a powerful tool for reconstructing long-term slip rates on active faults and can provide evidence for temporal earthquake clustering. Extensional faults are particularly amenable to this type of study because they commonly produce a striated bedrock scarp, exhumed by faulting, that can be sampled to obtain the concentration profile of a cosmogenic isotope as a function of fault-throw. Here we compare modelling results of a synthetic dataset of known slip history with that derived from a suite of in situ 36Cl concentration profiles sampled from carbonate normal fault scarps.
Existing methods for extracting paleo-earthquake records from such data use forward modelling and conclude that individual slip events >= 1m (>= Magnitude 7.0) may be resolved, although a cluster of smaller magnitude events can produce a similar 36Cl profile. Due to uneven scarp preservation sample spacing in real data sets is variable (up to 10s cm), further limiting our ability to extract tectonic information.
We use Monte Carlo inversion on synthetic 36Cl datasets to investigate the effect of sample spacing and analytical error on the interpretation of fault slip histories (i.e. number of earthquakes, timing, magnitude of slip) and show how sample density and analytical error influence our interpretation of the true slip history of a fault.
The conclusions drawn from our analysis of the synthetic data then guide our inversion of real 36Cl concentration profiles, derived from active normal fault scarps in Abruzzo, central Italy. We couple our data with LiDAR and sub-surface GPR to constrain the Holocene slip history of the faults and discuss the implications and limitations for deriving slip variability and earthquake hazards in the region.